Beam deflecting lasers



'1- 1- \IIVIIIII Aug. 29,

BEAM DEFLECTING LASERS Original Filed Dec. 19, 1962 FIGJ FIG. 2

VOLTAGE CONTROL INVENTOR. WILLIAM V. SMITH FIG. 10

ATTORNEY United States Patent O 3,339,151 BEAM DEFLECTING LASERS WilliamV. Smith, Chappaqua, N.Y., assignor to International Business MachinesCorporation, New York, N.Y., a corporation of New York Continuation ofapplication Ser. No. 245,702, Dec. 19, 1962. This application Jan. 12,1966, Ser. No. 532,820 8 Claims. (Cl. 331--94.5)

This application is a continuation of copending application Ser. No.245,702, filed Dec. 19, 1962, for lasers.

This invention relates to improved optical masers or lasers and, moreparticularly, to a device providing beam deflection and modulation ofthe output of an optical maser or laser.

The terms optical mas er and laser are intended to be synonymous termsand are used to describe devices which by stimulated emission produceradiation in the infrared, visible or ultra-violet portions of theelectromagnetic wave spectrum. The word maser is an acronym formicrowave amplification by stimulated emission of radiation. Whensimilar techniques are employed to produce waves within the opticalregion of the electromagnetic wave spectrum, the terms optical maser andlaser are used, laser being an acronym for light amplifications bystimulated emission of radiation. Optical devices of this type arecapable of producing radiation which is highly directional, coherent andmonochromatic.

In recent years, a large amount of research and development work hasbeen expended in attempting to develop masers and lasers as indicated bythe following patents and publications:

U.S. Patents 2,836,722, Atomic or Molecular Oscillator Circuit, and2,929,922, Masers and Maser Communications System.

Physical Review, vol. 112 page 1940, December 1958, Infared and OpticalMasers.

Nature, vol. 187, page 493, August 1960, Stimulated Optical Radiation inRuby.

This invention is particularly directed to a type of laser usuallyreferred to as a Fabry-Perot laser. In a Fabry- Perot laser, the lasercavity usually includes two parallel reflecting plates separated by asmall distance and enclosing a medium, capable of optical amplification.When supplied with a suitable source of pumping power, the mediumenclosed within the laser emits radiation in one or more modes. Such alaser will emit radiation at all wave lengths, or modes, which areintegrally related to the product of the index of refraction of themedium between the plates and the distance between the reflectingsurfaces.

Recently, it has been found that a laser cavity having greater modediscrimination can be constructed by replacing one of the reflectors ofthe cavity described above with a two-plate reflector. In such a device,a second, external, Fabry-Perot cavity is connected in series with thecavity which receives the pumping power. One reflecting surface of eachof the cavities forms a common interface between the two cavities.Greater mode discrimination is obtained by virtue of the fact that thefrequency at which phase coherence is maintained in the amplifyingmedium must also be a frequency for which the reflectivity of thetwo-plate reflector is high.

In such a laser, it is desirable to provide means to modulate or deflectthe beam output of the laser.

Accordingly, it is an important object of the present invention toprovide an improved optical laser of the type providing modulation orbeam deflection of the laser output.

It is a further object of the present invention to provide an improvedlaser of the type including an external cavity of an electro-opticalmedium, the index of refraction of which can be varied in order toselectively vary the emission of the primary laser cavity.

It is a further object of the present invention to provide an improvedoptical laser having an external cavity includ ing a plurality ofsub-cavities of electro-optical material, the index of refraction ofwhich can be varied to selectively vary the stimulated emission in theprimary laser cavity.

In accordance with one embodiment of the invention, a Fabry-Perot cavityis provided having two opposed reflecting surfaces enclosing anamplifying medium. A second, external, cavity is connected in serieswith the primary cavity. A reflecting surface of the primary cavity anda reflecting surface of the external cavity form a common interfacebetwen the two cavities. The external cavity is constructed of anelectro-optical material, the index of refraction of which can be variedby the application of an external electric field. This external cavityis divided into a plurality of contiguous rectangular parallelepipeds.Each of these parallelepipeds is a sub-cavity containing an electrodepassing through the longitudinal axis. The contact surfaces betweenthese parallel sub-cavities and the entire longitudinal surface of theexternal cavity is electroded and held at ground potential. Byselectively applying a suitable potential to each of the electrodes, itis possible to vary the index of refraction of each of the sub-cavities.The resulting inhomogeneity of index of refraction bends or dispersesthe laser radiations, thereby changing the reinforcement conditionssufliciently to quench the oscillations and to destroy the laserradiation in the primary cavity.

The foregoing and other objects, features, and advantages of thisinvention will be better understood from the following more detaileddescription and appended claims taken in conjunction with the drawings,in which:

FIG. 1 shows a representation of the present invention;

FIG. 1a shows and end view of the laser at the surface 2; and

FIG. 2 shows a practical embodiment of the laser of the presentinvention.

Referring to FIG. 1, the laser includes a primary resonating cavity 1having a partially reflective and partially transmitting surface at 2and a reflecting surface 3. The reflectivity of surface 3 is less thanthe reflectivity of surface 2. The primary laser is supplied withpumping radiation as indicated at 4 which stimulates emission to producethe coherent output at 5. The cavity 1 includes an active medium whichmay be made of calcium fluoride doped with trivalent uranium, aluminumoxide doped with chromium or calcium tungstate doped with neodymium.

Includes in series with the primary cavity 1 is an external cavity 6.The surface 3 forms a common interface between the cavity -1 and thecavity 6. The bottom surface of cavity 6 is a mirror forming one of thereflective surfaces of the cavity as is well known in the art.'Thereflectivities of all the reflective surfaces used in the inventionf-orm no part of the present invention, such reflectivities being wellknown and easily determinable by the requirement that the mirror lossesnot exceed the gain in the active medium.

The external cavity 6 is divided into a plurality of subcavities such asthose indicated at 7 and 8. Each of the subcavities is a parallelepiped.The contact surfaces between each of the parallel sub-cavities and theentire longitudinal surface, such as at 9 and 10, is electroded andconnected to ground potential. Passing through the longitudinal axis ofeach sub-cavity is a wire electrode such as the electrodes 11 and 12.Each of these wire electrodes may be held at a different voltage. Thecontrol voltage source 13 is provided for this purpose. The source 13applies a desired potential to each of the electrodes.

The external cavity 6 is constructed of an electro-optical materialhaving the property that the index of refraction varies in accordancewith the applied electric field. One example of an electro-opticalmaterial suitable for use in this invention is KDP.

The operation of the laser of the present invention is as follows: If avoltage is applied to all of the electrodes in each of the sub-cavities,the optical qualities of the external cavity are so impaired that thelaser does not oscillate. If one or more of these center electrodes aregrounded, there is no electro-optic deflection in that particularsub-cavity and a portion of the primary cavity 1 will oscillate. Theoscillating part can be switched by grounding the desired centerelectrode.

The reason for the above-noted impairment is believed to be thefollowing: When an electrical potential is applied to a centralelectrode, such as 11 or 12, the electrical field inside its associatedsub-cavity 7, 8, etc., is strongest near the center of such electrodeand weakens as the distance from the center of the electrode 11, 12,etc., increases. This latter condition results in a nonuniform index ofrefraction. The axial rays in cavity 1 that are incident to thesub-cavity through surface 3 strike the later with a plane phase front.As light waves enter the cavity 6, there occurs a slight change in theindividual wave length, A)\, of such entering axial rays. Since, for anygiven sub-cavity 7, 8, etc., to which a potential has been applied,there will be present in such sub-cavity a non-uniform index ofrefraction, so At will be different for the several axial rays whichform the phase front. Consequently, the phase front of the initiallyplane wave becomes bent, thereby destroying the reinforcement conditionsnecessary to sustain oscillation.

FIG. 1a shows the surface 2 of the laser through which the coherentmonochromatic output light 5 passes. By selectively energizing certainones of the electrodes in the sub-cavities, the laser emits light whichmay define a desired character. As shown in FIG. 1a, the laser isemitting light representing an X.

FIG. 2 is a diagrammatic representation of one embodiment of an opticallaser constructed in accordance with the principles of the presentinvention and operated at extremely low temperatures. In this figure,reference numerals corresponding to those used in FIG. 1 are used toidentify like components. The device includes the active cavity 1 andthe external cavity 6. The output of the laser is indicated at 5. Theprimary cavity and the external cavity are secured in a mounting 14located in the lower end of a long cylindrical tube 15. This tube 15 isdisposed in a Dewar vessel 16 having an outer container 17 and an innercontainer 18 which is partially filled with, for example, liquidnitrogen 19. A vacuum is maintained in the space 20 between the outercontainer 17 and the inner container 18. The mounting 14 for thecavities 1 and 6 is positioned in the lower end of the long cylindricaltube 15 so that the surface 2 of the cavity 1 through whichapproximately 1.5 percent of the incident light applied thereto istransmitted passes the coherent output light 5 through the longcylindrical tube 15. A vacuum is also maintained in the long cylindricaltube 15. Since the tube 15 passes through the liquid nitrogen 19 in theinner container 18, this tube 15 is essentially at 77 K. which is theboiling point for liquid nitrogen at atmospheric pressure. The KDPmaterial in the cavity 1 may be heated within a few degrees of its Curiepoint by any known means, such as the use of a heating coil.

The outer container 17 has a chamber 21 at the lower end thereof intowhich the cylindrical tube 15 extends and in which the mounting 14 isdisposed. The chamber 21 has a window or port 22 at one side thereof. Apumping light source 23 is surrounded by a light shield 24 so as todirect the pumping light through the window 22 of the chamber 21 to theouter surface of the cavity 1. The pumping light source 23 may be of anysuitable type providing either pulsed or continuous radiation, asdesired.

Examples of modifications of the laser of the present invention, whichare not to be considered as limiting, are as follows: While the outputlight 5 has been shown as passing through the upper surface 2 of theprimary cavity, the output light may also pass through the lower end ofthe external cavity. In this case, the surface at the lower end of theexternal cavity 6 should be partially reflecting and partiallytransmitting, surface 2 being a totally reflecting surface.

Although the laser has been shown as having a center reflectinginterface 3, this surface 3 need not be provided. The laser may beconstructed with only two mirrors or reflecting surfaces.

The laser of this invention has been shown in a practical embodiment asbeing of the low temperature type. This invention may also be used inconjunction with a gas laser. If the invention is used with a gas laser,it is possible to use barium titanate as the electro-optical materialfor the external cavity 6. Barium titanate may be used only inembodiments where the temperature is maintained a few degrees above theCurie point of barium titanate, which is approximately C.

While specific embodiments of the invention have been shown anddescribed, it will, of course, be understood that various changes may bemade without departing from the principles of the invention. Theappended claims are, therefore, intended to cover any such modificationswithin the true spirit and the scope of the invention.

What is claimed is:

1. A laser comprising an active element,

means for applying pumping radiation to said active element,

an external cavity connected in series with said active element,

and electrical means connected to said external cavity for selectivelydestroying reinforcement conditions necessary for oscillation inportions of said laser.

2. A laser comprising an active element,

means for applying pumping radiation to said active element, an externalelectro-optical cavity, said external cavity being connected in serieswith said active element,

and electrical means connected to said external cavity for causingselected portions of said external cavity to exhibit a non-uniform indexof refraction to destroy reinforcement conditions in portions of saidlaser.

3. The laser recited in claim 2 wherein the means for causing selectedportions of said external cavity to exhibit a non-uniform index ofrefraction includes means for selectively applying an electric field tosaid electrooptical material.

4. In a laser of the type including a first Fabry-Perot cavity having atleast two reflective surfaces, an active medium, and a source of pumpingradiation,

the improvement comprising an external Fabry-Perot cavity, said externalcavity being constructed of electro-optical material,

said external cavity having at least one reflective surface forming acommon interface with one reflective surface of said first Fabry-Perotcavity,

and means for selectively causing portions of said external cavity toexhibit a non-uniform index of refraction whereby light beams incidentto said portions are dispersed therein.

5. The laser recited in claim 4 wherein said external cavity includes,

a plurality of sub-cavities, each of said sub-cavities being aparallelepiped of electro-optical material, the surfaces of each of saidsub-cavities being electroded,

the surfaces of each of said sub-cavities being connected to a commonpotential,

each of said sub-cavities containing a wire electrode passing throughthe longitudinal axis,

and said last-mentioned means including means to selectively applypotentials to said wire electrodes.

6. A laser comprising:

a resonant cavity;

an active element disposed within said cavity;

means for exciting said active element to produce coherent oscillationsWithin said cavity;

structural means having a plurality of discrete side by side regions,each region being in series with said active element and in said cavity,means coupled to each region for altering the effective optical path ofsaid cavity including such region when actuating energy is appliedthereto; and

means for applying actuating energy to selected regions of saidstructural means to produce such changes in eiTective optical paths tothereby alter cavity reinforcement conditions necessary for producingsaid coherent oscillations in such selected regions.

7. A laser comprising:

a resonant cavity;

an active element disposed Within said cavity;

means for exciting said active element to produce coherent oscillationswithin said cavity;

a plurality of discrete side by side electro-optical regions, each ofwhich is in series with said active element and is disposed in saidcavity, means coupled to each region for changing its index ofrefraction when actuating energy is applied thereto; and

means for applying actuating energy to selected electrooptical regionsfor effecting changes in the index of refraction of such regions toalter cavity reinforcement conditions necessary for producing saidcoherent oscillations in such selected regions.

8. A laser as defined in claim 7 wherein said actuating References CitedUNITED STATES PATENTS 3/ 1966 Rigden et al. 33194.5 3/ 1966 Billings331--94.5

JEWELL H. PEDERSEN, Primary Examiner.

RONALD L. WIBERT, Examiner.

1. A LASER COMPRISING AN ACTIVE ELEMENT, MEANS FOR APPLYING PUMPINGRADIATION TO SAID ACTIVE ELEMENT, AN EXTERNAL CAVITY CONNECTED IN SERIESWITH SAID ACTIVE ELEMENT, AND ELECTRICAL MEANS CONNECTED TO SAIDEXTERNAL CAVITY FOR SELECTIVELY DESTROYING REINFORCEMENT CONDITIONSNECESSARY FOR OSCILLATION IN PORTIONS OF SAID LASER.